The present invention relates to a process for the production of aluminum by carbothermic reduction of alumina and to a reactor for the production of aluminum by reduction of alumina.
The direct carbothermic reduction of alumina has been described in U.S. Pat. No. 2,974,032 (Grunert et al.) and it has long been recognized that the overall reaction
Al2O3+3C=2Al+3COxe2x80x83xe2x80x83(1)
takes place, or can be made to take place, in two steps:
2Al2O3 +9C=Al4C3+6COxe2x80x83xe2x80x83(2) and,
Al4C3+Al2O3 =6Al+3COxe2x80x83xe2x80x83(3).
Reaction (2) takes place at temperatures below 2000xc2x0 C. and generally between 1900 and 2000xc2x0 C. Reaction (3), which is the aluminum producing reaction, takes place at appreciably higher temperatures of 2200xc2x0 C. and above; the reaction rate increases with increasing temperature. In addition to the species stated in reactions (2) and (3), volatile species including gaseous Al, gaseous aluminum suboxide, that is Al2O, and CO are formed in reactions (2) and (3) and are carried away with the off gas. Unless recovered, these volatile species will represent a loss in the yield of aluminum. Both reactions (2) and (3) are endothermic.
Other patents relating to carbothermic reduction to produce aluminum include U.S. Pat. Nos. 4,486,229 and 4,491,472 (Troup et al. and Stevenson et al.) Dual reaction zones are described in U.S. Pat. No. 4,099,959 (Dewing et al.). There, an Al2O3xe2x88x92Al4C3 phase diagram shows the conditions of temperature and composition for Grunert et al. reaction (5) to proceed at one atmosphere. Dewing et al. shows a variety of apparatus to produce aluminum where generally, the energy driving the reactions is supplied by resistance heating of slag in transit from a low temperature zone to a high temperature zone. Dewing et al. states that in most instances in his invention the aluminum-liberating reaction is carried out in an upwardly inclined passage and gas evolved is employed to achieve circulatory movement of the slag. There, molten slag of Al2O3+Al4C3 is generally continually recirculated.
The present invention relates to a process for carbothermic production of aluminum where aluminum carbide is produced together with molten aluminum oxide in a low temperature compartment. The molten bath of aluminum carbide and aluminum oxide flows into a high temperature compartment, where the aluminum carbide, that is Al4C3, is reacted with the aluminum oxide, that is Al2O3, to produce aluminum. The aluminum forms a layer on the top of a molten slag layer and is tapped from the high temperature compartment. The off-gases from the low temperature compartment and from the high temperature compartment, which contain Al vapor and volatile Al2O, that is aluminum suboxide, are reacted to form Al4C3. In the present invention, the low temperature compartment and the high temperature compartment are located in a common reaction vessel, with the low temperature compartment being separated from the high temperature compartment by an underflow partition wall. The molten bath containing aluminum carbide and aluminum oxide produced in the low temperature compartment continuously flows under the partition wall and into the high temperature compartment by means of gravity flow which is regulated by tapping of aluminum in the high temperature compartment. The energy needed to maintain the temperature in the low temperature compartment and in the high temperature compartment is provided by separate energy supply systems.
According to a preferred embodiment, the energy necessary to maintain the temperature in the low temperature compartment can be provided by means of high intensity resistance heating such as through electrodes submerged into the molten bath of aluminum carbide and aluminum oxide.
Similarly, the energy necessary to maintain the temperature in the high temperature compartment can be provided by a plurality of pairs of electrodes arranged in the sidewalls of that compartment of the reaction vessel.
According to yet another embodiment of the present process, the off- gases from the low temperature compartment and from the high temperature compartment are reacted to form Al4C3 which can be recycled.
The present invention further relates to a reactor for carbothermic production of aluminum, comprising a reaction vessel with a low temperature reaction compartment and a high temperature reaction compartment separated by a partition wall allowing underflow of molten bath from the low temperature reaction compartment to the high temperature compartment; a means for supplying alumina and a carbonaceous reduction material to the low temperature reaction compartment; a means for supplying alumina, aluminum carbide and/or carbon to the slag bath in the high temperature compartment; a means for supplying electric operating current independently to each of the low temperature reaction compartment and high temperature reaction compartment; and an over/underflow outlet for continuously tapping molten aluminum from the high temperature compartment.
Preferably, the means for supplying electric current to the low temperature reaction compartment is one or more electrodes that will effect the melting and reacting of the carbonaceous reduction material and the alumina; the electrode(s) are intended to be submerged in the molten bath in the low temperature compartment. More preferably, the electrodes in the low temperature compartment are graphite electrodes.
The means for supplying electric current to the high temperature reaction compartment is preferably a plurality of pairs of substantially horizontally arranged electrodes arranged in the sidewalls of that compartment; the electrodes will provide the heat necessary to produce aluminum, which will float to the top of the slag layer in the high temperature compartment.
According to a preferred embodiment, the reaction vessel has a substantially rectangular shape. Those portions of the reaction vessel, such as the bottom and the sidewalls, that are intended to be in contact with molten slag can be built up from a plurality of hot media-cooled panels that contain a xe2x80x9cfrozenxe2x80x9d slag layer on their sides facing the inside of the reaction vessel.
The present invention provides molten aluminum containing aluminum carbide, about 20 wt. % to 35 wt. %, and also includes cooling tapped molten aluminum to precipitate the aluminum carbide, followed by filtering, degassing and casting to form aluminum ingots.
Thus, the present process and apparatus provide a compact reaction vessel where the low temperature compartment and the high temperature compartment are integrated in one reaction vessel. In this manner, a xe2x80x9conce-throughxe2x80x9d process and apparatus are provided.
The present invention offers numerous advantages over the art. By injecting aluminum carbide and/or carbon into the high temperature compartment it is not necessary to return molten alumina slag from the high temperature compartment to the low temperature compartment.
The use of a plurality of pairs of sidewall electrodes in the high temperature compartment ensures that an even temperature is obtained in the slag in that compartment; this in turn results in a fast production of aluminum in the whole bath and avoidance of local superheating of the bath which would increase the amount of Al vapor and of volatile Al2O. Also, the side electrodes are below the molten aluminum layer rather than passing through it. This avoids/reduces localized superheating and resulting volatilization, and is an important part of the reactor design.
The aluminum tapped from the high temperature compartment is typically saturated with aluminum carbide and may thus contain between about 20 and about 35 percent by weight of aluminum carbide. By cooling the superheated aluminum tapped from the high temperature compartment, a major part of aluminum carbide contained in the aluminum will precipitate and can be skimmed off from the molten aluminum. This aluminum carbide is preferably recycled to the high temperature compartment. The remaining aluminum carbide contained in the molten aluminum after cooling to a temperature just above the liquidous temperature of the aluminum is recovered in a conventional way, such as by filtering of the molten aluminum.